1. Field of the Invention
This invention relates in general to a low abberation spectrometer objective having high electron acceptance.
2. Description of the Prior Art
Conventional scanning electron microscopes have beam blanking systems and are equipped with opposing field spectrometers and are currently utilized for quantitative measurements of potentials at nodes and interconnects in VLSI circuits. However, adequately fine electron probes for the examination of VLSI circuits having structures in the submicron range cannot be produced with modified scanning electron microscopes since these apparatuses must be operated at low primary electron energies so as to avoid radiation damage and charging of the components which are usually arranged in insulating carrier materials. A noticeable improvement in the spatial resolution which is essentially limited by the axial chromatic abberation of the objective lens and by the electronelectron interaction (Boersch effect) can be achieved only by means of a short electron optical beam path having few cross-overs and an objective lens having a short focal length. Up to the present, the use of short focal objective lens having a short working distance for the reduction of the chromatic and spherical image defects essentially defined by the focal length and working distance has been unsuccessful due to the structure of the conventional electron beam measuring instruments wherein a secondary electron spectrometer is arranged between the objective lens and the specimen.
Only as a result of the development of objective lens with integrated secondary electron spectrometer objective has the abberation of the objective lens been reduced with the working distance and, thus, the probe diameter on the specimen can now be demagnified. Such a spectrometer lens is described in publication by Kawamoto entitled "Electron Beam Tester With In the Lens Analyzer" published in the Proceedings of the Symposium on Electron-Beam-Testing dated Nov. 9-10 1984, Osaka, Japan Pages 69-72.
This known arrangement has a short focal length magnetic objective lens comprising an integrated parallel plate analyzer and an electrode arranged above the objective lens for deflecting the secondary electrons in the direction of a detector.
In this known spectrometer objective lens comprising a planar extraction and opposing field electrode however, no angle independent documentation of the secondary electrons triggered on the specimen and emitted into a larger solid angle ranges is possible and, thus, the obtainable resolution of potential is limited as the consequence of measuring errors which are thus caused.
U.S. Pat. No. 4,464,571 which issued on Aug. 7, 1984 entitled "Opposing Field Spectrometer For Electron Beam Mensuration Technology" in which the inventor is Erich Plies and which is assigned to the assignee of the present application discloses an improved electro-static opposing field spectrometer that can be utilized for taking measurements with an electron beam probe which has an extraction electrode and an opposing field electrode and two spherical networks for generating a spherically symmetrical opposing field.